Hands-free call systems for vehicle use an echo canceller to remove echoes caused by portion of a sound outputted from a speaker returning back into a microphone. The echo canceller acquires a voice signal to be outputted by the speaker as a reference signal, and removes the echo included in the signal inputted from the microphone (for example, refer to Patent Literature 1).
Based on an assumption that the outputs from a front left (FL) speaker mounted at a front left side area in a vehicle and a front right (FR) speaker mounted at a front right side area in the vehicle are symmetric, conventional echo cancellers acquire a reference signal from one of the FL and FR speakers. If the outputs from the FL and FR speakers are symmetric as assumed above, amounts of echo entering the FL and FR microphones, which are disposed left and right in the vehicle respectively, are equal. Therefore, echoes can be subtracted from input signals using an identical reference signal for both the FL and FR microphones. However, in a situation in which the outputs from the FL and FR speakers are asymmetric, for example, a voice guidance provided by a car navigation device is outputted only from a speaker on a driver side, the echo cannot be removed completely because amounts of echo entering into the microphones differ. Therefore, private information included in the voice guidance, such as currently traveling position, may be transmitted to a mobile phone at a far end.
To solve this problem, Patent Literature 2 proposes an echo canceller that mixes a sound signal to be outputted from an FL speaker and a sound signal to be outputted from an FR speaker to generate a reference signal, thereby adjusting the unbalance between the amount of echo entering an FL microphone and that entering an FR microphone.